Without a written employment contract requiring cause for termination, you are, in all likelihood, an at-will employee. As such, you serve at the whim of your employer. They can change your duties, compensation, schedule or even terminate for any reason or no reason at all.
The only exception is that any action taken against you may not be motivated by illegal discrimination (race, religion, disability, ethnicity, etc.) or sexual harassment.
If, upon further deliberation you think that may apply to your situation then you need to make a complaint to the EEOC as soon as possible.
Otherwise, without a claim, as described above, the courts view this as office politics and offer no judicial or administrative remedy. In other words, you would need to seek recourse within the HR department.